Biggest thing that wasn't very clear when starting out was understanding how gear abilities stack.
Talking to Judd (the cat announcer), he once said that you benefit from having variety in your gear because having multiples of a single skill isn't a direct multiplier. BUT I have learned from others on forums that this ONLY applies to the main abilities while sub abilities are unaffected.
So for example, if you have a hat and a shirt that both have a defense main ability, your total defense wont be increased by 2 rather only 1.5 ( Numbers are an example, I don't know what the actual stat value increase is). While that does increase defense more than a single main ability would, you're also missing out on having that ability slot being filled with something worth it's full value.
The way to get the most out of maxing an ability is utilizing the sub abilities. Each sub ability is worth 1/3 of a main ability, so if you have 3 of the same sub ability it is equal to 1 main ability even in addition to an already equipped main. So say you have that same defense hat, and you had a shirt that had 3 of its sub slots filled with defense sub abilities, you would have a total of + 2 in defense.
This concept is partially described in
this video. (I would highly recommend watching all the squid science videos as they all have useful info)
"How do I get my gear to have all the sub abilities I want?" you might ask. Well the assignment of sub abilities is, at it's core, random. The key does lie with obtaining the power to re-roll your stats once you reach level 20 from the urchin in the back alley, you can plan ahead when dropping the 30000 coins required to do so.
That lies with the brands. Each brand of clothing has advantages and disadvantages when rolling for sub abilities: (pulled this off a GameFAQs forum)
Amiibo,
Cuttlegear, and
KOG brands don't affect secondary abilities in any way.
Firefin - 5x chance for Ink Saver (Sub), 1/2 chance for Ink Recovery Sub
Forge - 5x chance for Special Duration Up, 1/2 chance for Ink Saver (Sub)
Inkline - 5x Defense Up, 1/2 Damage Up
Krak-On - 5x Swim Speed Up, 1/2 Defense Up
Rockenberg - 5x Run Speed Up, 1/2 Swim Speed Up
Skalop - 5x Quick Respawn, 1/2 Special Saver
Splash Mob - 5x Ink Saver (Main), 1/2 Run Speed Up
Squidforce - 5x Damage Up, 1/2 Ink Saver Main
Takoroka - 5x Special Charge Up, 1/2 Special Duration Up
Tentatek - 5x Ink Recovery Up, 1/2 Quick Super Jump
Zekko - 5x Special Saver, 1/2 Special Charge Up
Zink - 5x Quick Super Jump, 1/2 Quick Respawn
Hopefully this helped with planning out your ideal gear setup. Gear isn't everything, I would recommend building a solid playstyle with your weapon of choice and building your gear around that.
With the splatfests being this frequent and the payout on super sea snails high, you should be able to get a decent set without grinding.